Rotary printing machines, such as rotary offset lithographic presses, for example, utilize printing cylinders to which printing plates are conformed and secured. The printing plates rotate with the cylinders and apply the desired impression to a rubber covered blanket cylinder, which then transfers the impression to a sheet or web passed between the blanket cylinder and an opposing roll or plate. Typically, means are provided on the printing cylinder for mechanically engaging one or both ends of the printing plate in order to keep the plate properly positioned on the cylinder. Commonly, this is done by providing means for clamping the ends of the printing plate. In some cases, adhesive is utilized, alone or in conjunction with other means, to hold the printing plates in position.
It has been proposed heretofore to utilize vacuum means to retain the printing plates in position on the printing cylinders. Representative of prior proposals for this purpose are the Kamata, et al. U.S. Pat. No. 3,078,796 the Lake U.S. Pat. No. 3,112,698, the Musgrave U.S. Pat. No. 3,380,381, the Demaria, et al. U.S. Pat. No. 3,463,476 and the Johnson British Pat. No. 484,169. The theoretical advantages of the vacuum cylinder are evident, in that it is expected to be able to more easily mount printing plates thereon. Changing of printing plates is also expected to be facilitated. However, notwithstanding the expected advantages to be derived from the vacuum cylinder, its commercial acceptance has been extremely limited, because of the practical difficulties experienced in achieving an effective design of a vacuum cylinder suitable for use under conditions of day-to-day commercial production.
A greatly improved design and construction of a vacuum-operated printing cylinder suitable for utilization under conditions of typical commercial production has been proposed in copending Arkell application Ser. No. 504,254, filed Sept. 9, 1974 and owned by Livermore and Knight Co., Inc. In its various forms, the Arkell invention provides a highly effective and entirely practical printing cylinder capable of holding a printing plate by vacuum alone, without requiring auxiliary holding means. According to its teachings, vacuum power is effectively distributed to the cylinder surface by means of independently evacuable chambers within the cylinder. The separate chambers communicate with the cylinder surface via an effective arrangement of flow passages through the cylinder sleeve opening directly to the cylinder surface or to a series of annular grooves formed thereon. The utilization of independently evacuable chamber permits the application of vacuum holding power to areas of the printing cylinder actually covered by the printing plate with a minimum of leakage. This will accommodate the use of plates of varying sizes, as well as multiple plates on a side-by-side basis. In the latter case, some of the plates can be removed and replaced without affecting the other plates by maintaining the vacuum in certain chambers, while removing it from others.
As one of its basic objectives, the present invention seeks to provide a novel and improved, commercially acceptable vacuum-operated printing cylinder of the general type proposed in the co-pending Arkell application and which provides an instantaneous initial hold for the plates and facilitate a much shorter plate mounting time. Generally, the new vacuum-operated printing cylinder comprises an elongated supporting shaft extending from one end of the cylinder to the other end sufficiently beyond to provide bearing support. A hollow cylinder sleeve is secured to the supporting shaft by means of a plurality of cylinder supporting rings secured in axially spaced relation on the supporting shaft forming therewith one or more vacuum chambers within the printing cylinder.
As a unique feature of the present invention, the supporting shaft is formed to provide a pre-evacuation chamber. Several independently controlled valve means are built into the cylinder to provide vacuum communication between each individual vacuum chamber of the hollow cylinder and the pre-evacuation chamber. Vacuum communication between the vacuum chambers and the exterior surface of the printing cylinder is achieved by means of radial passages in the cylinder sleeve. The end of the pre-evacuation chamber is connected to piping which passes through the supporting shaft to a vacuum source.
The new and improved construction disclosed herein affords a highly advantageous means to apply effective vacuum holding power to the cylinder surface. The pre-evacaution chamber can be kept evacuated. When it it desired to attach a printing plate to the cylinder, the vacuum power of the pre-evacuation chamber will be available to be immediately applied to the exterior surface of the printing cylinder by operating the appropriate valve means. The result is a virtually instantaneous hold on the plate, freeing the operator to perform other tasks. Moreover, since the pre-evacuation chamber partially evacuates the vacuum chambers the time required to build up to a running hold to retain the plate on the cylinder during operation of the printing press will be greatly diminished.
Consequently, the inventive concept of a pre-evacuation chamber greatly enhances the overall efficacy of a vacuum-operated printing cylinder. Moreover, by forming a cylindrical pre-evacuation chamber within the supporting shaft itself, the pre-evacuation chamber provides a good structural component for the printing cylinder and affords easy construction.
In accordance with another form of the present invention, vacuum communication between the pre-evacuation chamber and the outer surface of the printing cylinder can be achieved to provide an effective balance with respect to the amount and distribution of exposed vacuum area. Adequately distributed vacuum holding power can be made available to secure the printing plates, while at the same time minimizing leakage, which would reduce the overall holding effectiveness of the cylinder.
More particularly, the printing cylinder surface is provided with a plurality of continuous annular grooves. The annular grooves are interrupted by longitudinally extending grooves, continuous from one end of the cylinder to the other. A soft metal strip is then tightly received in the longitudinal grooves to effectively block off the annular grooves and provide a predetermined circumferential discontinuity between adjacent ends of each groove. Preferably, four longitudinal grooves are cut into the cylinder sleeve to divide the cylinder surface into four circumferential regions.
Each discontinuous annular groove communicates with a radial passage in the cylinder sleeve. The radial passages are arranged so that all the radial passages of a predetermined longitudinal section of the printing cylinder communicate with one of a plurality of axial bores formed in the cylinder sleeve. Each axial bore is provided with manually accessible valve means for selective vacuum communication with the pre-evacuation chamber.
To particular advantage, this arrangement allows the vacuum power from the pre-evacuation chamber to be immediately, selectively applied to a large number of predetermined cylinder surface areas. If, for example, four longitudinal grooves are provided and four axial bores are provided for each of the four sets of discontinuous annular grooves formed thereby, the printing cylinder surface will be divided into sixteen predetermined areas. Consequently, if a small printing plate is to be attached to the cylinder all the vacuum power can be directed to the area covered by the plate. The otherwise exposed areas of the cylinder surface will be isolated from the pre-evacuation chamber by the action of the independent valve means to minimize leakage.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description and accompanying drawings.